An American endurance swimmer has attempted to become the first person to swim from Havana to Key West in Florida without the aid of a shark cage.

Diana Nyad is hoping to accomplish at 61 years old what she failed to do at 28, and if successful, would become the first person to traverse the 100-mile Florida strait without a cage. She will instead rely on technology and divers to fend off predators.

Tanned and freckled from long hours training in the open seas of the Caribbean, Nyad expressed confidence before starting off just before sunset Sunday. She said the still air and calm water were perfect conditions for her attempt to make the 103-mile (166km) swim. "The adrenaline's flowing now. This is what I dreamed – a silver platter."

She gave a kiss on the cheek to the commodore of the Hemingway Marina in Havana, who helped arrange the logistics of the trip. Then she changed into a black swimsuit and blue swim cap and goggles, which were light blue – for better night vision – and smoky charcoal-tinted to protect her eyes from the blinding daytime sun.

After an assistant greased her shoulders and armpits to prevent chafing in the salty water, Nyad played Reveille on a bugle, thanked several dozen wellwishers who came to see her, then jumped feet first into the sea.

Nyad said it has been a lifelong dream and she hopes her feat, if successful, will inspire people to live vigorously during their golden years. "Thirty-three years ago I stood on a beach close by here and looked out at a giant sea ... Now I'm almost 62 years old and I'm standing here at the prime of my life," she said. "I think this is my day."

Nyad first had a go at this crossing back in 1978, when she swam inside a steel shark cage for about 42 hours before sea currents hammering her off course put an end to the attempt.

The following year she set a world record for open-water swimming without a shark cage, charting 102.5 miles (165km) from the Bahamas to Florida, then retired from competitive endurance swimming.

Nyad said the aborted Cuba attempt had stuck with her all these years and upon turning 60, she started thinking about a comeback. "What if I went back and tried to chase that elusive dream of Cuba?" she said. "I started training and found it was in my heart and it was in my body ... It seems almost like a dream to me, but now it's real."

In preparation, she has made eight- to 15-hour swims over the past two years off the Caribbean island of St Martin.

Australian swimmer Susie Maroney successfully swam the shark-filled waters from Cuba through the Florida Straits and to the Keys in 1997, although she used a cage.

Nyad's team is deploying an electronic boom to surround her with a current that is imperceptible to humans but is strong enough to keep most sharks at bay. Whitetip sharks are not deterred by the field, so divers will be standing by to gently discourage the creatures without harming them.

For the record to be considered valid, Nyad will have to make the swim without a wetsuit. Her crew will navigate, monitor her health and provide nourishment. But she is not allowed to touch the boat, nor can her helpers hold her, until she's fully emerged fully on to dry land.

Five support vessels carried 45 navigators, nutritionists, doctors, shark wranglers and a film crew that has been documenting her story.

She plans to stop every 45 minutes for 20-second hydration breaks of water, juice and sports drinks. Every 90 minutes she will rest for two minutes and nibble on bread or a spoonful of peanutbutter.

By day two she'll begin drinking heated water and hot chocolate to ward off hypothermia, which is a genuine threat after so many hours at sea, even though water temperatures are expected to be 30-31C (86-88F).

Nyad embarked on a north-west course, aiming to arrive at a point in the keys slightly east of due north, compensating for sea currents. She called the attempt a "symbolic moment" for increasing understanding between the US and Cuba, two nations torn by five decades of animosity and mistrust.

"I'm under no delusion that my swim is going to make any new political ramifications," Nyad added. "But it is a human moment between the two countries."